Description

Historically, lithography referred to a method of transferring an images or text from a flat stone surface, where they were previously created, onto paper. Currently, the term "lithography" is used in a broad sense as a technology of image transfer.

With regard to nanotechnology the term “lithography” often refers to the technology of microelectronics that includes several stages:

2) drying and irradiating (exposing) the film coating on the wafer with a specific pattern through an appropriate mask;

3) developing (etching) the exposed coating in a special solution;

4) forming a physical structure of electronic components on the substrate.

In the last decade, the term "lithography" has been used in a broader sense to refer to the method of forming not only electronic circuits, but also nanostructures (or images with nanometre resolution) on a substrate by transfer of images using a mask or a stamp, or by direct effect on the sample surface (lithography using STM or AFM).

The following methods of lithography are distinguished: optical, ultraviolet, X-ray, electron beam and ion-beam lithography. Holographic interference lithography can be carried out using light, ultraviolet, or synchrotron X-ray radiation.

Scanning tunnelling microscopy (STM) makes it possible to perform a number of nanolithographic operations, such as surface modification, transfer of the probe material onto the sample and vice versa, which makes it possible to create lithographic patterns with nanometre resolution.

Lithography can also be carried out using atomic force microscopy (AFM). In this approach the microscope probe moves on the surface of the substrate with a sufficiently large pressing force, so that a pattern in the form of depressions (scratches) is formed on the substrate (or overlying resist). Pressing on the probe can be replaced by the supply of a current pulse to the sample (in this case the impact area is melted or partially evaporated). This method of lithography (nano-etching) has several advantages over electron/ion-beam lithography; for example, there is no need for additional process steps (etching, etc.). However, it also has some disadvantages; thus, under static action of the probe accidental torsional bending of the cantilever can lead to edge discontinuities at the image, and the scanning operation (preceding and following the nanolithographic operation) leads to shear distortions of the image.

In the case of dynamic AFM lithography, depressions are formed by a vibrating probe during scanning in the intermittent contact mode. This method solves the problem of image distortion (and allows the pattern to be visualised without significant impact on the surface). Such lithography can be performed using vector or raster scanning (the first method enables high speed of the process, but it does not make it possible to vary the force of impact onto the substrate during lithography). One of the important varieties of AFM lithography is the anodic oxidation lithography; it makes it possible to change not only the geometric characteristics of the surface, but also its local electrical properties (application of bias voltage to the conductive cantilever stimulates electrochemical processes on the surface directly under the sample, that can be accompanied by oxidation of metal layers, see anodizing). The probe and the sample surface act in such a process as a cathode and an anode; and the thickness of the grown anodic oxide can be varied by changing the applied electrical potential.

Another currently popular type of lithography is the imprint lithography; it uses a stamp with a nano-relief, which plays the same role as the template in contact optical lithography. The stamp is produced by the method of electron-beam lithography and anisotropic plasma-etching. The nanorelief is "imprinted" in the polymer covering the substrate in the conditions of high temperature and high pressure. The polymer with the nanorelief serves as a mask in subsequent operations (etching, implantation, etc.). This method demonstrates the possibility of creating structures with a record resolution and density. The resolution value achieved by this method is ca. 6 nm and the distance between elements of the structure is 20-30 nm. The main limitation of the method is the difficulty of aligning the patterns to create different layers of the structure.